Phosphors are compositions that are capable of emitting useful quantities of radiation in the visible, infrared and/or ultraviolet spectrums upon excitation of the phosphor compound by an external energy source. Due to this property, phosphor compounds have long been utilized in cathode ray tube (CRT) screens for televisions and similar devices, as taggants for authenticating documents and products, and as luminescent coatings in fluorescent lamps, x-ray scintillators, light emitting diodes, and fluorescent paints. Typically, inorganic phosphor compounds include a host lattice containing a small amount of an emitter or dopant ion.
Conventional phosphor compositions have a defined response (e.g., emission) to a given excitation by electromagnetic radiation. Phosphor compositions may, for example, convert one wavelength to a second wavelength whether ultraviolet to visible, ultraviolet to infrared, visible to visible, visible to infrared, infrared to infrared, or infrared to visible. Phosphor compositions may also emit near the wavelength they absorb.
When used in security applications, phosphor compositions are applied to articles to be authenticated and some basic property of the phosphor, generally the wavelength of the emitted radiation, is monitored by a detector to provide an indication or “tag” of whether an article being tested in genuine. In some applications, it is necessary to differentiate between a series of tagged articles and, although this can be achieved by applying different phosphor compositions which emit radiation of different wavelengths, this has the disadvantage of requiring a monitoring system with multiple detectors capable of separating and reading the different wavelengths.
For certain applications, including differentiation between a series of tagged articles, it would be advantageous to have phosphor compositions that can controllably emit different responses, e.g., emissions, to a single or multiple energy sources. For example, U.S. Published Patent Application No. 2009/0007815 discloses a particulate composition having a first response to a first electromagnetic radiation and, after intermediate exposure to a second electromagnetic radiation, a second response to the first electromagnetic radiation, different from the first response.
According to the present invention, a multiphase photosensitive particle is provided, in which the same particle contains multiple different phases, each producing a different response when excited by radiation of the same energy or different energies so that the overall output of the particle is equal to the sum of the responses between the phases. By varying the relative amounts of the different phases it is possible to produce a series of particles with different outputs useful in security applications requiring authentication and differentiation between articles.